What is Post-Quantum Cryptography (PQC)?

Post-quantum cryptography, usually shortened to PQC, refers to cryptographic algorithms designed to remain secure when large-scale quantum computers become available. It addresses a specific and growing problem: most public-key cryptography in use today relies on mathematical assumptions that quantum computing is expected to break. PQC replaces those assumptions with alternatives that are believed to hold even in a quantum computing context.

The objective of PQC is not speculative or academic. It is a direct response to known weaknesses in widely deployed cryptographic systems once quantum capabilities mature. For organisations that rely on encryption, digital signatures, and identity infrastructure, PQC represents a necessary evolution of cryptography rather than a new security model.

Why current cryptography has limits

Modern digital security depends heavily on public-key cryptography. Algorithms such as RSA, Diffie–Hellman, and elliptic-curve cryptography underpin TLS, VPNs, software updates, email encryption, and public key infrastructure. Their security is based on problems like integer factorisation and discrete logarithms, which are infeasible to solve with classical computers at realistic scales.

Quantum computing changes this assumption. A sufficiently capable quantum computer can use Shor’s algorithm to solve these problems efficiently, making it possible to recover private keys from public keys. When that happens, encryption can be decrypted, and digital signatures can be forged. This is not a matter of weaker security margins but of fundamental breakage.

Symmetric cryptography is affected differently. Quantum algorithms such as Grover’s algorithm reduce the effective security of symmetric keys, but this can be mitigated by using larger key sizes. Public-key cryptography does not have this option. It requires new algorithms built on different mathematical foundations.

What post-quantum cryptography changes

Post-quantum cryptography replaces vulnerable public-key algorithms with ones based on problems that are not known to be solvable efficiently by either classical or quantum computers. These problems come from different areas of mathematics, such as lattices, error-correcting codes, and hash functions.

From an operational perspective, PQC does not require quantum hardware. The algorithms run on standard servers, endpoints, and network devices. The change sits in cryptographic libraries, protocols, certificates, and key management processes rather than in physical infrastructure.

This distinction is often misunderstood. PQC is not cryptography for quantum computers. It is cryptography for classical systems that must remain secure in a world where quantum computers exist.

From theory to standardisation

The move towards PQC is already well underway. In response to the quantum threat, the US National Institute of Standards and Technology launched a multi-year process to evaluate and standardise quantum-resistant algorithms. After extensive cryptanalysis and industry input, NIST published its first set of PQC standards in 2024, covering key establishment and digital signatures

This milestone shifted PQC from research into implementation. Once standards exist, vendors can integrate them into products, protocols can be updated, and organisations can begin structured migration planning. The publication of standards also signals that the timelines are no longer abstract. Cryptographic transitions typically take many years, which is why action is required well before large-scale quantum computers become practical.

The relevance of “harvest now, decrypt later”

One of the main drivers behind PQC adoption is the risk known as harvest now, decrypt later. Adversaries can capture encrypted traffic today and store it until quantum capabilities allow decryption. This is especially relevant for data with long confidentiality requirements, such as intellectual property, health records, government communications, and industrial data.

In this context, the security of encrypted data depends not on when it is decrypted, but on when it was encrypted. If vulnerable algorithms were used at the time of encryption, the data may already be compromised from a future perspective. PQC directly addresses this risk by ensuring that encrypted data remains protected even if intercepted years earlier.

Why is PQC becoming a necessity?

PQC is increasingly treated as a baseline requirement rather than an optional enhancement. Major technology providers are already testing or deploying post-quantum algorithms in controlled ways. Standards bodies and governments are setting expectations for PQC readiness across critical infrastructure and public sector systems

For organisations, this means PQC needs to be understood in the same way as earlier cryptographic transitions, such as the move from SHA-1 to SHA-256 or from short RSA keys to stronger configurations. The difference is scale. PQC affects nearly every system that relies on public-key cryptography, and many of those systems are deeply embedded and long-lived.

Setting the foundation for what comes next

This first blog establishes what post-quantum cryptography is and why it exists. It deliberately avoids deployment detail and algorithm mechanics. Those topics follow naturally once the underlying problem is clear. PQC  is one of the practical building blocks within a broader quantum security landscape, alongside areas such as cryptographic agility, governance, and longer-term developments.

The next step is understanding the quantum threat itself in more depth: which algorithms fail, how quickly, and why timelines matter. This blog explains why PQC is not a distant concern but a present planning requirement.

Post-quantum cryptography is best understood as managed change. It is not an emergency response, but it is also not something that can be deferred until quantum computers arrive. The organisations that treat PQC as part of their long-term cryptographic lifecycle will find the transition controlled and predictable. Those who do not will face compressed timelines and limited options later.